Traditionally, lubricating oils for automatic transmissions, manual transmissions, and internal combustions engines have been required to be improved in various durabilities such as thermal oxidation stability, anti-wear properties and anti-fatigue properties, and viscosity temperature characteristics and low temperature viscosity characteristics for improving fuel efficiency, such as reduced low temperature viscosity and low temperature fluidity. In order to improve such characteristics and properties, a lubricating oil has been used, whose base oil is appropriately blended with various additives, such as anti-oxidants, detergent-dispersants, anti-wear agents, friction modifiers, seal swell agents, viscosity index improvers, anti-foaming agents, colorants, and the like.
Recent transmissions and engines have been demanded to be fuel efficient, small and light and be increased in power output. Transmissions have been sought to be improved in power transmission capability in connection with the increased power output of the engines with which the transmissions are used in combination. Therefore, lubricating oils to be used for such transmissions have been demanded to maintain high lubricity and to possess properties to prevent the wear or fatigue of the surfaces of bearings and gears while reduced in the product viscosity and base oil viscosity. Further, automatic transmissions and continuously variable transmissions are supposed to be used in a cold region of −10° C. or lower and are thus required to be further improved in low temperature performance so as to enhance the low temperature startability and improve the fuel efficiency at low temperatures. Generally, a technique is employed, wherein viscosity temperature characteristics are improved by reducing the base oil viscosity and increasing the amount of a viscosity index improver in order to improve fuel efficiency. However, a reduction in the base oil viscosity degrades anti-fatigue properties. Therefore, the development of a lubricating oil has been eagerly desired, which can achieve both good fuel efficiency and anti-wear properties or anti-fatigue properties at higher levels at the same time. An improvement in low temperature viscosity characteristics can be accomplished by reducing the base oil viscosity or the final product viscosity. However, it has been known that a reduction in the base oil viscosity degrades anti-wear properties and anti-fatigue properties. The development of a lubricating oil has been eagerly desired, which can achieve both low temperature viscosity characteristics and anti-wear properties or anti-fatigue properties.
Under these circumstances, it has been known to use a base oil with a good low temperature performance or a base oil with a high viscosity in combination or to blend a phosphorus- or sulfur-based extreme pressure additive in a suitable amount in order to improve fuel efficiency, low temperature viscosity characteristics and anti-fatigue properties (see, for example, patent documents 1 to 3 below).
However, the techniques disclosed in these patent documents are not sufficient to attain viscosity temperature characteristics, low temperature performance and anti-fatigue properties, and load bearing properties at the same time. Therefore, it has been demanded to develop a lubricating oil composition having all of these performances and characteristics but undergoing no problem in other performances.                Patent Document 1: Japanese Patent Laid-Open Publication No. 2004-262979        Patent Document 2: Japanese Patent Laid-Open Publication No. 11-286696        Patent Document 3: Japanese Patent Laid-Open Publication No. 2003-514099        